Certain metals commonly referred to as shape memory alloys exhibit characteristic material properties that make them desirable for use in actuators. Shape memory alloy actuation provides greater force per volume than electromagnetic-type actuation, and is also less complex. These characteristics make shape memory alloy actuation highly desirable for use in fuel injectors, particularly automotive fuel injectors.
Shape memory alloys (hereinafter "SMAs") undergo a temperature-related phase change which is characterized by the memory of any mechanical configuration imposed on the material in its austenitic crystalline phase. In particular, SMAs have two different crystal structures that are determined by temperature. In its low temperature state the material exhibits a martensitic crystal structure which has a relatively low modulus of elasticity, and which can be easily deformed. However, when the alloy is heated above a temperature threshold, the transition temperature, its crystal structure changes to austenite and the alloy returns to its original configuration.
This temperature-dependent memory characteristic is exploited in actuators for fuel injectors by providing a bias mechanism, for example a spring, to deform the SMA element while it is in the low temperature state, then raising the SMA element's temperature, for example by resistance heating, in order to induce a return to the element's original configuration. The return to the SMA element's original conformation thereby creates motion in the spring, which in conjunction with the remainder of the actuator assembly results in opening or closing of the fuel injector valve. Cooling of the SMA element returns the element to its low temperature, easily deformed phase. The bias spring force results in mechanical motion in the actuator which closes or opens the fuel injector valve. A major challenge in the use of SMAs in automotive fuel injectors has been to reduce the response time of the alloy so that the opening or closing cycle of the actuator is reduced to one millisecond or less. This fast response time is required in order to provide the necessary minimum flow control necessary under light load engine conditions.
It is known in the art that the response time is affected by the rate of heat transfer (i.e., cooling) of the SMA element, and that the geometry of the alloy element has a direct affect on this heat transfer rate. SMA actuator geometries comprising small-diameter wires, ribbons, or thin films, for example, have been shown to maximize the heat transfer rate of the alloy, thereby achieving faster response times. Such geometries have been described in U.S. Pat. No. 4,806,815 to Homma; U.S. Pat. No. 4,973,024 to Homma; U.S. Pat. No. 5,061,914 to Busch, et al.; U.S. Pat. No. 5,211,371 to Coffee; and U.S. Pat. No. 5,325,880 to Johnson et al. The width-to-thickness ratios disclosed in the prior art are in the range from 50:1 to 4:1, and resulted in best minimum response times of about 10 milliseconds. However, none of these geometries yield the requisite degree of heat transfer effective to provide response times at the 1 millisecond level required for fuel injector applications.
It is further known in the art that the response time is affected by the energy input (e.g., resistance heating) into the SMA element. Ordinarily, a high energy input into the SMA element is desirable, in order to decrease the response time. This energy input has an inherent limitation, however, due to the nature of the materials suitable for shape memory alloys. An "over-temperature" condition results in strain recovery loss or destruction of the alloy. The response time of SMA actuators in the prior art have accordingly been limited in the amount of input power which may be applied to the SMA elements, and again, are limited to response times of no less than 10 milliseconds. There thus remains a need in the art for economical methods and apparatus for controlling the operating conditions of shape memory alloy actuators for fuel injectors so as to provide response times of less than 10 milliseconds, and preferably less than about 1 millisecond.